The long-term goal of this research is to understand how multisensory inputs interact in the control of human upright stance to provide an empirical-theoretical basis of the postural consequences of central or peripheral sensory deficits. In order to remain upright, it is crucial to know the relative position of different body segments, including the center of mass, relative to the surrounding environment. How we construct such an internal model is unknown, but is thought to be heavily dependent upon multisensory information about our own body segments and the surrounding environment. The important question of how the various sensory inputs interact for postural control is largely unresolved. We propose to simultaneously manipulate visual and somatosensory stimuli, and analyze the coupling between multisensory information and postural sway. The specific aims are: 1. To establish that multisensory integration is based on forming stable spatio- temporal relationships between postural sway and multisensory information.. We will manipulate the temporal and spatial structure of visual and somatosensory information simultaneously and study the coupling between multisensory stimuli and postural sway. Is the coupling dependent on mode (vision vs. somatosensation) or on properties of the sensory input (spatial, temporal)? 2. To study how sensory fusion of multisensory information is achieved in postural control. We will experimentally "pull apart" two sensory inputs by presenting each with a different temporal structure. We will then analyze the coupling between postural sway and multisensory information to observe how multisensory information is fused or how a single sensory source may be selected. 3. To study the process of postural adaptation to multisensory information. Postural responses to an abrupt change in frequency of visual or somatosensory inputs alone will be compared to a change in both stimuli together. How does the postural control system adapt to multisensory information as opposed to single stimuli? What is the time course of that adaptation? 4. To study the influence of task dependence on multisensory integration. We will study how the coupling of postural sway to multisensory information is influenced by instructions to detect movement of either visual or somatosensory cues. The question is how such attention changes the perceptual quality of the stimulus and consequently the postural response to that stimulus.